1. Field of the Invention
The present invention relates to a liquid ejecting head and an ink jet printing apparatus which eject a liquid for printing, and in particular, to a liquid ejecting head that inhibits sub-droplets from being generated during ejection.
2. Description of the Related Art
An ink jet printing system is known as a common scheme of ejecting a liquid such as ink to print a print medium. The ink jet printing system includes a method of utilizing electrothermal converting elements (heaters) as ejection energy generating elements that allow the liquid to be ejected and a method of utilizing piezoelectric elements (piezo). Both types of elements are provided in a liquid ejecting head and can control ejection of droplets in accordance with electric signals.
To meet the recent demand for high-image-quality printing, much effort has been made to reduce the size of ejected droplets and to increase the number of nozzles provided in the liquid ejecting head. Consequently, the adverse effect, on printing, of droplets which are different from those ejected for printing and which do not contribute to printing is no longer negligible. Specifically, during ejection, droplets are separated into main droplets and sub-droplets (hereinafter also referred to as satellites). The main droplets impact the desired place on a print medium. However, the impact positions of the satellites cannot be controlled. With conventional low-image-quality printing, the satellites have almost no adverse effect on printing. However, with the present high-image-quality printing, the printing image quality may be markedly degraded by the satellites.
Furthermore, smaller satellites may lose speed before reaching the print medium and become floating ink droplets (hereinafter also referred to as mist). The mist may stain the printing apparatus. The stain on the printing apparatus may be transferred to the print medium, which may thus be stained.
To prevent printing image quality from being degraded, Japanese Patent Laid-Open Nos. 9-239986 and 10-235874 disclose a method of reducing the generation of satellites by forming noncircular ejection openings.
The shape of the ejection openings described in Japanese Patent Laid-Open Nos. 9-239986 and 10-235874 enables a reduction in the generation of satellites. However, when the noncircular ejection openings are designed to eject the same amount of liquid as that of corresponding circular ejection openings for comparison, the noncircular ejection openings are likely to be subjected to a greater flow resistance and thus inappropriate ejection because of the longer circumferential length thereof. In particular, a phenomenon is likely to occur in which ejection through the noncircular ejection openings becomes difficult a specified time after the start of ejection.
As described above, the reduction in the generation of satellites may be contradictory to the maintenance of the easiness with which ejection can be performed the specified time after the start of ejection. On the other hand, the generation of satellites and the easiness with which ejection is performed the specified time after the start of ejection also depend on the volume of ink (hereinafter sometimes simply referred to as the ejection amount). That is, even with the same shape of the ejection openings, the amount of satellites generated and the easiness with which ejection can be performed the specified time after the start of ejection may vary depending on the type of the ink. The generation of satellites and the easiness with which ejection can be performed the specified time after the start of ejection may also vary depending on the ejection amount.